This invention relates generally to radio frequency amplifier circuits and more particularly to low noise radio frequency amplifier circuits.
As is known in the art a radar system is one particular type of radio frequency (RF) system which detects RF signals. The radar system generally includes an antenna, a transmitter and a receiver. In a transmit mode, the radar transmitter emits an RF signal from the antenna. Portions of the transmitted RF signal which are intercepted by an object (e.g. a target) and reflected back toward the radar are generally referred to as an "echo" or "received" signals.
The antenna intercepts a portion of the echo signal having an RF frequency and feeds such signals to the receiver. The receiver detects RF signals fed thereto and provides an intermediate frequency (IF) signal at an output port. The IF signal is subsequently fed to a digital signal processor for further signal processing as is generally known.
Noise is unwanted RF energy which interferes with the ability of the receiver to detect the echo signal. The capability of the receiver to detect a weak echo signal is limited by the noise energy that occupies the same portion of the frequency spectrum as does the echo signal.
Noise energy may enter the radar receiver from external sources along with the echo signal via the antenna. Noise energy may also originate in the receiver itself due to various causes such as thermal motion of conduction electrons in ohmic portions of those circuit components which are disposed to provide the receiver. The receiver cannot detect the presence any signal having a power level below the power level of the noise energy. The weakest signal the receiver may detect is generally referred to as the minimum detectable signal. Thus, the power level of the noise energy is said to "set" the power level of the minimum detectable signal.
In the absence of a so-called jamming signal, external sources provide relatively low power levels of noise energy. Thus the noise energy provided by the radar receiver limits the power level of the minimum detectable signal.
The effectiveness of the receiver to detect echo signals in the presence of noise energy may be represented by a figure of merit generally referred to as the noise figure of the receiver. In general, the noise figure of a radar receiver may be defined as the input signal to noise ratio divided by the output signal to noise ratio. The input signal to noise ratio is provided by the ratio of the input signal power to the input noise power. The output signal to noise ratio is provided by the ratio of the receiver output signal power to the output noise power. Thus, the noise figure may be interpreted as a degradation of the input signal to noise ratio as the echo signal passes through the receiver.
In the ideal case, the receiver should not degrade the input signal to noise ratio. Therefore, in the ideal case, the noise figure of the receiver is unity (i.e. 0 decibels).
Every circuit component disposed to provide the receiver contributes to the noise figure of the receiver. The composite noise figure of N circuit components connected in cascade may be calculated from Equation 1. ##EQU1##
In Equation 1, F.sub.cas is the noise figure of the cascade connection, F1 and G1 are the noise figure and power gain respectively of the first component in the cascade connection, F2 and G2 are the noise figure and power gain of the second component, etc. . . . Note that the term G.sub.1 (i.e. the gain of the first component) appears in the denominator of all of the terms following the term F.sub.1. Thus, the contribution to the cascaded noise figure F.sub.cas of those circuit components following the first component is reduced by the power gain G.sub.1 of the first component.
A radar system having a so-called low noise receiver generally includes a low noise amplifier (LNA) and a mixer connected in cascade. The LNA is provided having a high power gain characteristic G.sub.1 and a noise figure F.sub.1 close to unity. The mixer is provided having a power gain characteristic G.sub.2 which, in general, is less than unity and a noise figure F.sub.2. Thus, to provide a receiver having a relatively low noise figure the LNA is provided as the first active component of the receiver.
Because of the low noise figure and high gain characteristic of the LNA those circuit components which follow the LNA have little effect on the noise figure of the receiver. Thus the LNA is said to "set" the overall noise figure of the receiver.
Conventional low noise amplifiers include two or three discrete transistors such as field effect transistors for example, connected in cascade. One particular type of field effect transistor having low noise characteristics is the so-called high electron mobility field effect transistor (HEMT). Thus a low noise amplifier is provided by cascade connecting such HEMTs.
In general HEMTs have respectively input and output impedances which differ significantly from the characteristic impedance of transmission lines coupled to the LNA. Therefore, HEMTs generally require input and output impedance matching networks for each amplifier stage. Thus, in the LNA several of such amplifier stages (i.e. HEMT and input matching network and output matching network) are cascade connected together.
Several problems exist with this approach. First, the input and output impedance matching networks may be relatively complex and further such networks have a relatively high insertion loss characteristic. High insertion loss will reduce power gain and increase the noise figure of the LNA. To compensate for losses in the matching network, one could bias the HEMT to provide high gain and thus provide an overall gain for the amplifier.
However HEMT transistors when biased to provide a relatively high power gain characteristic provide a potentially unstable amplifier stage. That is, a particular impedance characteristic provided to the input or output port of the HEMT amplifier stage having a high power gain characteristic could cause the HEMT amplifier stage to become unstable and oscillate. Thus, to provide a practical LNA comprised of HEMT amplifier stages a design trade-off is made between the power gain characteristic and the stability requirement of the amplifier stage. This trade off thus results in an overall lower power gain or higher noise characteristic for the amplifier circuit.
Furthermore, in some radar system applications such as missile guidance radar systems where a limited amount of space is available, such low noise amplifier circuits must be provided in a relatively small package size. Moreover, in expendable systems such as missile guidance radar systems a relatively low cost amplifier circuit is desirable since the missile explodes at the end of its flight. Also, such circuits should be easily manufactured using current state of the art manufacturing techniques to provide reliable circuits having electrical characteristics which do not vary substantially among several of such circuits.
Although monolithic microwave integrated circuits (MMICs) satisfy the small package size required of circuits used in a missile guidance radar system several problems exist with a complete MMIC circuit approach. First MMIC circuits are relatively expensive. Moreover, in those applications requiring both low noise figure and high gain, use of a solely MMIC circuit approach may not provide an amplifier having the requisite gain and noise figure requirements. Thus, given the size, cost and manufacturing requirements of a missile guidance radar system it is relatively difficult to provide a RF amplifier circuit having a low noise figure and a high gain characteristic.